Method of coating aluminum



Patented Feb. 14, 1939' UNITED STATES PATENT orrice This application is METHOD OF COATING James W. Newsome, New Kensington, and Janina D. Edwards, Oakmont, Pa.., minum Company of America, a corporation of Pennsylvania assignors to Alu- Pittsburgh, Pa,

No Drawing. Application July 6,1938,

Serial No. 217,730

' cclaims. (01. 148 -6) a continuation-in-part of our copending application, Serial No. 121,012, for

Method of coating aluminum,

filed January 16,

This invention relates to the production of oxide coated aluminum surfaces, and it is particularly concerned with a method for the production of a substantially colorless, hard,,adherent and adsorptive coating consisting substantially of aluminum oxide on an aluminum surfaceby a nonelectrolytic chemical treatment. The term aluminum as used herein and in the appended claims is intended to include both aluminum in its various degrees of purity and the aluminum base alloys.

Oxide coatings have found extensive application for the protection and decoration of aluminum surfaces. This field has been extended particularly in recent years as a result of the development of the so-called anodic oxidation methods in which hard, adherent, abrasion resistant coatings are formed on aluminum by electrolytic oxidation. These coatings are generally more or less porous and adsorbent, and canbe colored, impregnated with corrosion inhibiting substances, sealed, or otherwise treated, as their particular use demands. Howev er, these methods do not;

lend themselves readily to the production of oxide coatings'on small articles, such as buttons, eyelets, and the like, and

their use is not economical in the production of certain types of articles intended for sale at relatively low prices.

While it has been recognized that the methods of producing'coatings on aluminum by simple.

for general use. In order to be satisfactory for.

general use, it is desirable that the coating produced should be to resist the abrasion sumciently hard and adherent incident to ordinary use,

and should'preferably be substantially colorless and more or less adsorbent, so that it can be colored by dyeing or pigmentillg.

It is an object of .this chemical treatment method of an oxide coating on bination of properties, as a finishing coating. object of the production of a this invention invention to provide a aluminum having a. compermitting its general use It is more particularly an to provide a method for hard, adherent, adsorbent erin, and silicates.

' erin may be used over a solution that little or for the production.

hours may cause sepa and substantially colorless coating comprising aluminum oxide on an aluminum surface by simple chemical treatment without the aid of an electric current. 1 e

We, have discovered that a relatively hard, ad- 5 herent, adsorbent and substantially colorless oxide coating can be produced on aluminum and aluminum alloy surfaces by treatment of the surface with an alkali aluminate solution containing a substance which stabilizes the solution against 10 precipitation of aluminum hydroxide by hydrolysis of the alkali aluminate. The treatment is preferably carried out-in a sodium aluminate solution containing any one of the class of stabilizers comprising tannic acid, oieic acid, glucose, glyc- 15 The article to becoated is preferably immersed in the stabilized alkali metal aluminate solution at an elevated temperature.

' The reaction between the coating solution and the metal surface, whichis at first fairly vigorous, 20 gradually becomes slower and finally stops, generally within about twenty minutes when operated at temperatures of to centigrade. The coating produced is relatively hard and adherent, and is more-or less adsorbent and substantially 25 colorless.

Satisfactory results may be obtained by using aluminate solutions containing as little as 0.2 per cent of sodium aluminate or as much as 6 per cent or even more, but for most purposes a concentra; 30 tion of 1 to 2 per cent has proved satisfactory. The amount of stabilizer used will vary with the particular stabilizer employed. When sodium silicate, tannic acid, and oleic acid are used as stabilizers, they are preferably present. in small amounts such as 1 to 2 per cent of the amount of sodium aluminate in the solution. when using glucose, a somewhat greater proportion relative to the amount of sodium'aluminate'is desirable. Good resultshave been obtained using glucose in an amount equal to about 10 to 25 per cent of the amount of sodium aluminate present. Glycrelatively wide range of concentration with substantially equivalent results, but we prefer to us'eit in the solution in quantity equal to the amount of sodium aluminate present. It is important in preparing the no'alumina should separate from the sodium aluminate and precipit'ateas aluminum hydroxide. Heating an unstabilized solution above 50 centigrade, or standing at room temperature, for a period of in excess of a few.

preferable thereforeto dissolve the stabilizer first and then dissolve the aluminate in the stabilizer 55 tion of alumina.. It is r solution. Alumina separates slowly, if at all, from I the stabilized solution even on longstanding, provided it is protected from carbon' dioxide.

In view of the fact that the sodium aluminate solution is super-saturated with respect to alumina, it requires some attention to keep it working properly. During use, additional alumina sodium aluminate to the point that the coating action of the solution is destroyed, and it becomes an etching action. This difficulty may be avoided .by adding to the sodium aluminate solution, during use, additional small amounts of stabilizer to prevent the seeding eflect of the precipitated alumina.

The temperature of the treatment is not critical. The coating operation proceeds most rapidly at elevated temperatures, and an operating temperature of 70 to 100 is preferred. A good coating is obtained within this range of temperature in about 10 to 30 minutes. As the temperature is lowered, the time required to produce a good coating increases. A good coating may be obtained at room temperature over a period of- 10 to 20 hours.

Preliminary cleaning of the aluminum surface to be coated is not essential, though it may be desirable. We have found that aluminum surfaces on which there is a film of grease may be satisfactorily coated when treated according to our invention, without any preliminary treatment to remove the grease. It is generally preferred, however, to use a clean aluminum surface. In some cases it may be preferred to preliminarily etch the surface to be coated. Particularly good results may be produced on surfaces preliminarily etched in hydrofluoric acid, followed by washing with strong nitric acid.

The properties of the coating obtained by the method of ourinvention will vary somewhat with the composition of the aluminum or aluminum alloy surface treated, and the exact conditions of treatment used. Certain alloying constituents,

such asiron and manganese, have a tendency to impart color to the coating formed, particularly if the coating solution is of relatively high concentration. It is preferable, therefore, when coating articles formed of aluminum alloys containing sirable color in the coating. Certain of the stabilizers which are themselves colored, such as tannic acid, also tend to impart color to the coating. It is generally preferred, therefore, tame a colorless stabilizer in the coating solution, and we have found that sodium silicate gives the most satis-.

factory results.

Ihe coatings obtained are generally more or less adsorptive, and can be colored by dyes. 'Ihe adsorptive properties of the'coating are generally .such that the colors obtained by dyeing are the lighter, pastel shades.

As a specific example of the methodof coatin aluminum according to the process of our invention, we may cite the following: The aluminum to be coated was heated in a solution containing -l per cent of-sodium aluminate and .01 per cent ofsodium silicate. The sodium aluminate had the approximate composition NaAlOz. The sodium silicate was a commercial brand in which the soda and silica were in the ratio of about 1 to ,3. The solution was maintained at a temperature of about centigrade, and the treatment continued fora period of about 20 minutes. There was a visible-reaction between the aluminum and the solution at the start of the treatment, as evidenced by the liberation of gas at the surface of the metal. This decreased, however, and at the end of the treatment time, all visible action had apparently ceased. After treatment the aluminum was-found to be coated with a hard, adherent coating having a thickness, as determined by microscopic examination of a cross section, of about 0.0002 inch. This coating was substantially colorless.

As an example of the method of our invention applied to an aluminum alloy surface containing an alloying constituent having a tendency to color the coating formed, the following treatment is cited: An aluminum alloy containing125 percent manganese and 1.0 per cent magnesium was first immersed in a solution containing 0.5 per cent sodium aluminate and 0.005 per cent sodium silicate at a temperature of about centigrade. This treatment was continued. for a period of about 10 minutes. The coated surface obtained from this treatment was then immersed in a coating solution containing 2 per cent sodium aluminate and 0.03 per cent sodium silicate at a. temperature of about 90 centigrade.

This treatment was continued until the action stopped. There was obtained by this double treatment a substantially colorless, hard, adherent, adsorbent coating on the metal surface.

The following is a further example of the practice of our invention: The aluminum to be coated was immersed for a period of about 29 minutes in a solution containing 1 per cent of sodium aluminate and 0.3 per cent of glycerin, at a temperature of 90 centigrade. At the beginning of the treatment, the liberated gas at the surface of the metal gave evidence of a reaction between the aluminum and the solution. This liberation of gas decreased, however, and all visible action had ceased at the end of the treatment. The alumi num was thereby provided with a hard, adherent, adsorbent, substantially colorless coating which was found, upon microscopic examination of a cross section, to be about 0.000092 inch in thickglucose in an amount equal to 10 to 25 per cent adsorbent, substantially colorless oxide coating on an aluminum surface, comprising treating said surface with a solution containing sodium aluminate in an amount equal to 0.2 to 6 per cent, and glycerine.

3. A method of producing a hard, adherent, adsorbent, substantially colorless oxide coating on an aluminum surface, comprising treating said surface with a solution containing an alkali aluminate in an amount equal to 0.2 to 6 per cent, and glucose in an amount equal to 10 to 25' per cent of said alkali aluminate present.

4. A method of producing a hard, adherent, adsorbent, substantially colorless oxide coating on an aluminum surface, comprising treating said surface with a solution containing sodium aluminate in an amount equal to 0.2 to 6 per cent, and

of said sodium aluminate present.

5. A method of producing a hard, adherent, adsorbent, substantially colorless oxide coating on an aluminum surface, comprising treating said surface with a solution containing an alkali aluminate in an amount equal to 0.2 to 6 per cent, and glycerin in an amount equal to the amount of said alkalialuminate present.

6. A method of producing a hard, adherent, adsorbent, substantially colorless oxide coating on an aluminum surface, comprising treating said surface with a solution containing sodium aluminate in an amount equal to 0.2 to 6 per cent, and glycerin in an amount equal to the amount of said sodium aluminatepresent.

- JAMES W. NEWSOME.

JUN'IUS D. EDWARDS. 

